The present invention relates to what is called a knock control system (KCS) for detecting a knocking generated in an internal combustion engine and controlling knock control factors such as ignition timing and air-fuel ratio.
Generally, a knock decision (discrimination) level V.sub.KD is prepared by multiplying an average value V.sub.mean of knock sensor signals by a constant K. Such a knock control system poses the problem that the optimum value of the constant K changes dependently on differences of sensors involved in manufacture of them or with lapse of time, thereby making impossible accurate knock detection. In order to solve this problem, a system has been invented for correcting a knock decision level automatically on the basis of the form in which the knock sensor signal is distributed. The disclosures of U.S. Pat. No. 4,617,895 and JP-A-No. 62-267574 are herein incorporated by reference.
Even such a system, however, has the problem that the knock decision is made improperly before the decision level is corrected. The present inventors have studied the reason why accurate knock detection is impossible by the conventional systems of decision level preparation.
As indicated by U.S. Pat. No. 4,617,895, a knock intensity value V (an effective quantity for detecting a knock, such as a maximum peak value within a predetermined section of a knock sensor signal) is based on the logarithmic-normal distribution. FIG. 1 shows a distribution of the knock intensity value V in the total absence of knock as plotted on the logarithmic-normal probability paper. In the Figure, characters (a) and (b) represent assumed distributions with small and large standard deviations (which may be replaced by the variances) of log(V) respectively. The standard deviation of log(V) is naturally subject to changes with differences of sensors involved in manufacture or with lapse of time. The standard deviation of log(V) is referred to because the standard deviation of the knock intensity value V is substantially meaningless among those assuming a logarithmic-normal distribution. This is also the case with the average value V.sub.mean and the median value V.sub.M. These two values are completely different and coincide with each other only in normal distribution. The value V.sub.M is more significant for a knock sensor signal in a logarithmic-normal distribution. In the description that follows herein, therefore, a knock decision level will be prepared on the basis of the more significant value V.sub.M. If a decision level is prepared as V.sub.D =K.times.V.sub.M (K=2), for example, the probability of knock decision for (a) is 1% while the figure is 10% for (b). As a result, in an engine having a distribution like (a), knock decision is rarely made in the absence of a knock, and therefore the ignition timing is controlled substantially to the largest advance angle in a system in which the ignition timing is controlled by detecting presence or absence of a knock. For an engine having a distribution like (b), however, its false knock decision cannot be ignored as the ignition timing would be delayed leading to a torque loss even when no knocking has occurred. Apart from the foregoing cases where no knocking occurs, the inconvenience is that a knocking, if occurred, may not be decided as so. In a system of decision level preparation using V.sub.KD =K.times.V.sub.M or V.sub.KD =K.times.V.sub.mean, therefore, accurate knock detection is impossible for different standard deviations of log(V). Specifically, such a system only can meet the situation in which the magnitude alone of the knock sensor signal undergoes a change and does not take the standard deviation of log(V) into consideration.
The technique suggested in JP-A-No. 62-267574 is capable of meeting the change in the variance of the log(V) distribution. Specifically, in this system, the knock decision level is prepared as V.sub.KD =A.times.D.times.V.sub.M (D: Constant), where A is a quantity subject to change in accordance with the variance of the log(V) distribution. The change in variance due to variations of engine or knock sensor can thus be absorbed.
Since the frequency at which the value A is updated is almost the same as that for knock decision, however, the updating rate is low, with the result that it takes a long time before a knock decision level is corrected to an optimum value.